1. Field of the Invention
The present invention relates to a broadcast receiver which receives the airwaves of predetermined channels transmitted from a plurality of broadcast stations, by a directionality switching antenna.
2. Description of the Related Art
In Japan, in general, the airwaves of television broadcasts or the likes are transmitted from a base station in one place. Therefore, substantially all of the airwaves which a plurality of broadcast stations transmit can be received in such a way that an antenna for receiving the broadcasts is fixed to the roof, the veranda or the like of a house so as to look toward the base station, and that the antenna and a broadcast receiver are connected by a cable. In contrast, in, for example, U.S.A., airwaves are individually transmitted from a plurality of broadcast stations, respectively. Therefore, when an antenna is fixed looking toward one place, the airwave of the broadcast station located in a direction in which the antenna looks can be received, but the airwave of the broadcast station located in a direction in which the antenna does not look cannot be received.
Accordingly, the directionality of one antenna must be switched in a plurality of directions in order that the airwaves from the plurality of broadcast stations located in different bearings may be received by the antenna. Such an antenna capable of switching its directionality (hereinbelow, termed “directionality switching antenna”) is stated in, for example, JP-A-7-226611 (paragraphs 0019-0025, FIGS. 1 and 3), JP-A-11-298226 (paragraphs 0008-0010, FIG. 5), and JP-A-62-69733 (lines 9-17, left upper column, page 3, and line 16, right lower column, page 3—line 14, right upper column, page 4, FIGS. 1 and 4) mentioned below. The directionality switching antenna includes a plurality of antenna elements, and phase shifters, etc. corresponding to the individual antenna elements. It adjusts the phases of signals received by the respective antenna elements, by the corresponding phase shifters, and then combines the resulting signals, thereby to electrically vary the directionality. Regarding a smart antenna which is one example of the directionality switching antenna, it is stipulated by standards to switch the directionality in 16 directions.
When such a directionality switching antenna is employed, the airwave from each broadcast station can be received in such a way that a control signal is transmitted from a broadcast receiver to the antenna so as to switch the directionality of this antenna to a predetermined direction. Thus, even when a user has fixedly mounted the directionality switching antenna on the roof or the like of a house, he/she can receive almost all airwaves transmitted from a plurality of dotted broadcast stations.
Methods of controlling the directionality switching antenna are disclosed in JP-A-7-226611, JP-A-11-298226, and JP-A-62-69733. In JP-A-7-226611, several appropriate parameters are stored in a storage circuit, whereby a pointing direction can be scanned at will in accordance with a pointing-direction control signal, and a directionality pattern can be simultaneously changed in accordance with the pointing direction by appropriately selecting the parameter. In JP-A-11-298226, each time a reception channel is altered, is the directionality of the antenna is scanned in all bearings, reception power levels on that occasion are measured, and the directionality of the antenna is set at the bearing affording the maximum reception power level, whereby the directionality of the antenna can be optimally set every channel. In JP-A-62-69733, when a reception state has become inferior, the beam direction of a combined pointing characteristic is swung around a beam direction on that occasion, while an angle is being gradually widened alternately counterclockwise and clockwise, and reception is continued in a direction in which the reception state has become favorable.
In a broadcast receiver which receives a airwave by a directionality switching antenna as stated above, when a frequency-locked state has missed, the state of unreceivability appears in which the airwave having been received till then fails to be normally received. In such a case, it is considered that, as shown in FIG. 8 by way of example, the pointing direction of the antenna is successively scanned from direction-0, which is the pointing direction in the case of the unreceivability, to direction-15, so as to detect a receivable direction. Since, however, this method examines the proprieties of reception in all directions, it is inferior in efficiency.
Besides, in JP-A-62-69733, in a case where unreceivability has occurred, a favorable reception state is searched for in such a way that the directionality of the antenna is switched around a pointing direction on that occasion, while an angle is being widened alternately counterclockwise and clockwise. More specifically, the directionality is switched 45° clockwise from an initial position (0°), and in a case where reception is inferior at the resulting position, the directionality is switched 45° counterclockwise from the initial position (0°). In a case where reception is inferior even at the resulting position, the directionality is switched 90° clockwise from the initial position (0°), and in a case where reception is inferior even at the resulting position, the directionality is switched 90° counterclockwise from the initial position (0°). Further, in a case where reception is inferior even at the resulting position, the directionality is switched 135° clockwise from the initial position (0°). In this manner, the directionality is switched up to 180° counterclockwise and clockwise, respectively, while the angle is being gradually widened. It is also stated in this document to randomize the switching direction of the directionality.
However, this document is premised on an antenna device which is disposed in a vehicle or the like mobile object. In such a device, the reception state frequently fluctuates depending upon the moving direction of the mobile object. In the antenna device for such a mobile object, accordingly, it is somewhat significant for quickly finding the receivable direction to switch the directionality in a wide range over the whole circumference or at random as in JP-A-62-69733. In contrast, in case of a broadcast receiver which is disposed indoors, an antenna device to be connected therewith is fixedly mounted on the roof or the like of a house, and hence, the reception state rarely fluctuates as frequently as in the case of the mobile object. Therefore, when the directionality of the antenna is switched by the method as in JP-A-62-69733 upon the occurrence of the unreceivability, a rather longer time is sometimes expended till the detection of the favorable reception direction. Any practicable means for solving such a problem is not indicated in JP-A-7-226611 and JP-A-11-298226, either.
Accordingly, an object of the present invention is to provide a broadcast receiver which can efficiently detect a receivable direction when unreceivability has occurred in a directionality switching antenna.